Single-pass optical fiber amplifiers consist of a length of rare-earth doped optical fiber in which the optical signal to be amplified is propagated. An optical coupler is used to couple a pump light in the rare-earth-doped optical fiber to pump the rare-earth-doped medium. The signal may either co-propagate or counter-propagate with the pump light, or the rare-earth-doped fiber may be pumped from both sides. Isolators are typically placed at both ends of the rare-earth-doped optical fiber.
Long-band optical amplifiers are amplifiers used to amplify a signal with a wavelength that is offset from the peak of the emission cross-section of the rare-earth dopant used. For example, long-band erbium-doped amplifiers are used to amplify optical signals in the L-band, i.e. 1570 nm to 1620 nm, while erbium has a peak emission at about 1530 nm. In another example, long-band ytterbium-doped amplifiers are used to amplify optical signals with wavelengths of 1064 nm and above while the ytterbium peak emission is at about 1030 nm.
Long-band amplifiers typically uses absorption of Amplified Spontaneous Emission (ASE) generated in the rare-earth-doped fiber as a result of pumping to further enhance the gain at the wavelength of the optical signal to be amplified. This principle usually requires a relatively long length of rare-earth-doped fiber since the absorption cross section at the ASE peak emission is quite lower than the absorption cross section at the pump wavelength. U.S. Pat. No. 6,222,670 to Ryu et al. and U.S. Pat. No. 6,233,092 to Flood et al. teach long-band amplifier architectures that uses ASE generated in one amplification stage as a result of traditional pumping, to pump another amplification stage.